What if things go wrong?

• Just like in a communication system, errors may occur.
• A speck on the envelope may turn a short dash into a long dash.
• Grease on the evelope may prevent the ink from sticking, and a long dash can become a short dash.

  

Figure 2.19: Incorrect USPS bar code.

• In the example above, the 6-th digit is in error.
• This can be determined, because that code word contains only one long dash.
• If we denote the unknown value of the 6-th digit by e, then we know that
  2+2+0+3+0+e+4+4+4+0+0+7=26+e.
• Since the check sum must be a multiple of 10, we can determine that e=4.
• We say, that the USPS bar code is capable of correcting one bit error.